Mysterious boxes

I keep getting this box that pops up really fast, doesn't last for more than a second and disappears. Kind of like a pop up advertisment that is killed before it has a chance to appear?

if you log out eachtime and if you get back, there is a pop-up yes

Yep. Pop-ups are enabled for unregistered users. But they shouldn't close themselves.

Do you have a popup blocker?

If not then you might have spyware installed on your PC. I suggest you take a look at: http://www.lavasoftusa.com/software/adaware/

thanks, I'll check it out.